Technical Field
The present invention relates to an absolute encoder including a reading device that reads a plurality of relative movement amounts from a plurality of respective incremental patterns provided on a scale, the absolute encoder synthesizing the plurality of relative movement amounts to acquire absolute position information.
Background Art
Conventionally, an encoder has been known that includes a scale having an incremental pattern (INC pattern) consisting of graduations arranged in parallel along a measurement direction, and a head having a reading device that moves along the scale and detects a relative movement amount from the INC pattern. Examples of encoder types include an electromagnetic induction type, a capacitance type, and a photoelectric type.
Explaining the electromagnetic induction type as an example, an electromagnetic induction type encoder includes a scale having scale coils (graduations) arranged in parallel along a measurement direction, and an index scale (head) relatively movable along the scale. The head has a transmitting coil facing the INC pattern of the scale, and a receiving coil (reading device).
The electromagnetic induction type encoder detects a relative movement amount of the head to the INC pattern from change in magnetic flux detected by the receiving coil via the scale coil when the transmitting coil is excited.
In such an electromagnetic induction type encoder, for example, an electromagnetic induction type ABS encoder described in JP 2005-345375 A has three types of INC patterns respectively having graduation array pitches different from each other.
Wavelengths detected from the respective INC patterns are different from each other, and a fine pattern from which the shortest wavelength is detected, a coarse pattern from which the longest wavelength is detected, and an intermediate pattern from which an intermediate wavelength between the fine pattern and the coarse pattern is detected are arranged in parallel.
The electromagnetic induction type ABS encoder, first, synthesizes an absolute position (a synthesized absolute position), based on an electric signal read by the reading device from the INC patterns at the time of turning on a power supply, and initially sets the synthesized absolute position as a preset value. Next, during measurement, the electromagnetic induction type ABS encoder performs an arithmetic operation between the preset value and a relative movement amount (relative position) based on the electric signal read by the reading device from the fine pattern to calculate an incremental count (a calculated absolute position), and outputs the calculated absolute position as a measurement value. In addition, the electromagnetic induction type ABS encoder synthesizes relative positions respectively based on the intermediate pattern and the coarse pattern to generate the synthesized absolute position. Then, the electromagnetic induction type ABS encoder compares the calculated absolute position based on the fine pattern with the synthesized absolute position based on the intermediate pattern and coarse pattern, to perform the comparison operation for determining whether or not there is an error between the calculated absolute position and the synthesized absolute position.
In this way, the electromagnetic induction type ABS encoder performs the comparison operation between the calculated absolute position and the synthesized absolute position, thereby improving reliability of the measurement value.